A new analysis of errors in the perceptual evaluation of spatial orientation suggests that these may be due to cognitive biases around categorical frames of reference. Spatial orientation is a fundamental component of a broad range of spatial cognitive behaviors from ambulatory navigation to interpreting a radiograph. Preliminary studies of the perception of spatial orientation show that for slanted lines and surfaces explored visually and for slanted surfaces explored by touch, perceived orientation is biased away from the perceived horizontal and toward the perceived vertical. The specific shape of the bias function is somewhat different for 2D line orientation than for 3D surface orientation, but the biases are in the same direction. The proposed studies seek to establish the extent of the biasing roles of conceptual reference frames such as vertical and horizontal. The first study will examine the flexibility of the assignment of reference frames by rotating observers so that the major axis of the body is no longer aligned with the gravitational vertical. It is anticipated that perceived orientation of a line whose orientation is intermediate between the gravitational vertical and the bodily vertical will be distorted in a different direction (but always toward the vertical) depending on which vertical participants are instructed to use as the reference frame for evaluation. Further studies seek to test for category-based biases for surfaces and lines confronted at an angle rather than straight ahead - both by vision and by touch - and to examine whether the biases found in the perceived orientation of small- scale objects viewed within reach are simply amplified for large-scale surfaces such as hills or are added to visual biases in hill perception. The cognitive exaggeration of deviations from horizontal may add to the misperception of hills as steeper than they are for the sake of making behaviorally-important information more accessible for the control of action. PUBLIC HEALTH RELEVANCE: Spatial cognition is related to a wide range of cognitive and perceptual matters relevant to fundamental health issues including preventative behavior (avoiding accidents and falls), medical diagnosis (in the interpretation of radiographs, for example) and mobility training for persons with disabilities (in the evaluation of accessibility of ramps or steep paths, for example). The proposed project studies cognitive biases in perceived orientation that may affect health outcomes as described above.